Frodo peering forward saw in the distance two great rocks approaching: like great pinnacles or pillars of stone they seemed. Tall and sheer and ominous they stood upon either side of the stream. A narrow gap appeared between them....

As Frodo was borne towards them the great pillars rose like towers to meet him. Giants they seemed to him, vast grey figures silent but threatening. Then he saw that they were indeed shaped and fashioned: the craft and power of old had wrought upon them, and still they preserved through the suns and rains of forgotten years the mighty likenesses in which they had been hewn. Upon great pedestals founded in the deep waters stood two great kings of stone: still with blurred eyes and crannied brows they frowned upon the North. The left hand of each was raised palm outwards in gesture of warning; in each right hand there was an axe; upon each head there was a crumbling helm and crown. Great power and majesty they still wore, the silent wardens of a long-vanished kingdom. Awe and fear fell upon Frodo.... Even Boromir bowed his head as the boats whirled by... under the enduring shadow of the sentinels of Númenor. So they passed into the dark chasm of the Gates....

Sheer rose the dreadful cliffs to unguessed heights on either side.... The black waters roared and echoed, and a wind screamed over them. Frodo... heard Sam in front muttering and groaning:... 'What a horrible place! Just let me get out of this boat...!'

'Fear not!' said a strange voice behind him. Frodo turned.... In the stern sat Aragorn son of Arathorn, proud and erect...: a king returning from exile to his own land....

'Long have I desired to look upon the likenesses of Isildur and Anárion, my sires of old.'

After... Atanatar the Glorious the Northmen... became powerful. Though these people... were usually friendly, they now became restless. [Rómendacil II] was forced to withdraw his northern border east of Anduin to the Emyn Muil. He there built the Gates of Argonath with images of Isildur and Anárion beyond which no stranger was allowed to come south without leave.

The Peoples of Middle-Earth, HoME Vol 12, Part 1, Ch 7, The Heirs of Elendil: The Southern Line of Gondor: The Anárioni

[Said Théoden,] 'But... we must still think of our frontier to the north and east.... So great a power as the Dark Lord seems now to wield might well contain us in battle before the City and yet strike with great force across the River away beyond the Gate of Kings.'

ar(a)- 'high, noble, royal' appears in a great many names, as Aradan, Aredhel, Argonath, Arnor, etc.; extended stem arat- appearing in Aratar, and in arato 'champion, eminent man', e.g. Angrod from Angaráto and Finrod from Findaráto; also aran 'king' in Aranrúth. Ereinion 'scion of kings' (name of Gil-galad) has the plural of aran; cf. Fornost Erain 'Norbury of the Kings' in Arnor. The prefix Ar- of the Adûnaic names of the Kings of Númenor was derived from this.

The Silmarillion, Appendix: Elements in Quenya and Sindarin Names

gond 'stone' in Gondolin, Gondor, Gonnhirrim, Argonath, seregon. The name of the hidden city of King Turgon was devised by him in Quenya as Ondolindë (Quenya ondo = Sindaringond, and lindë 'singing, song'); but it was known always in legend in the Sindarin form Gondolin, which was probably interpreted as gond-dolen 'Hidden Rock'.